1. Field of the Invention
The present invention relates to the systems on bicycles used to shift between gears, wherein such systems include index-type gear shifter levers and derailleur assemblies that are interconnected by cables. More particularly, the present invention relates to devices that can adapt the use of a set of indexing gear shifter levers with dissimilar derailleur assemblies on a custom built bicycle.
2. Prior Art Statement
Many bicycles contain a drive mechanism that has multiple gears. By shifting the bicycle""s chain between the gears, the mechanical advantage between the pedals and the rear wheel of the bicycle can be selectively altered. Traditionally, the mechanism used to shift the bicycle""s chain between different gears contains three interconnected assemblies. These assemblies are the gear shifter levers, the derailleurs and the cables that extend between the gear shifter levers and the derailleurs. The gear shifter levers are the levers that are positioned on or near the handlebars of the bicycle. These levers are moved by the person riding the bicycle, thereby providing manual shifting control to rider. The derailleurs are the mechanism that guide the bicycle chain onto different gears and compensate for slack in the chain as the chain moves between gears of different diameters. The cables are the elements that interconnect the shifter levers and the derailleurs and transfer mechanical movement between the gear shifter levers and the derailleurs. The cable is a direct mechanical drive. Consequently, if a gear shifter lever is used to move a cable ⅝th of an inch, the cable transfers that ⅝th of an inch of movement directly to a derailleur.
Because cables directly interconnect the gear shifter levers and the derailleurs, the gear shifter levers and the derailleurs must be coordinated into matched sets. For example, if an indexing-type gear shifting lever is designed to shift gears for every xc2xc inch of movement it causes in a cable, a derailleur must also be designed to shift gears for every xc2xc inch of movement it receives from the cable. If the gear shifter levers and the derailleurs are not coordinated, movement of the gear shifter levers between gears will not correspond to the proper derailleur movement required to shift gears. The result is that the desired gear shifting will not occur or may only partially occur, thereby creating chain slippage or chain chatter between gears.
Many bicycling enthusiasts customize their bicycles. Often gear shifter levers of a first type are desired to be used with derailleurs of a second type. However, since the gear shifter levers and the derailleurs are not compatible, the two assemblies cannot be used together. In the prior art, devices have been designed that mechanically alter the movement of a cable between a gear shifter lever and a derailleur. Such prior art devices are exemplified by U.S. Pat. No. 5,447,475 to Socard, entitled Device And Process For synchronous Control Of Bicycle Derailleurs. However, such prior art devices are intended to convert the movement of one cable into two cables, or visa versa. Such prior art systems do not enable dissimilar gear shifter levers and derailleurs to be used.
A need therefore exists for a device and method that can enable dissimilar indexing shifting levers and derailleurs to be used on a custom bicycle. This need is met by the present invention as described and claimed below.
The present invention is a device and method of mechanically synchronizing the displacement requirements of bicycle derailleurs with the displacement created by index-type gear shifter levers. The device includes a linkage element that is mounted to a bicycle between the gear shifter levers and the derailleurs. The linkage element is mounted to the bicycle with a pivot hinge, wherein the linkage element is free to rotate about the pivot hinge. A cable from each gear shifter lever is attached to a linkage element at a first distance from the pivot hinge. A cable from each derailleur is attached to a linkage element at a second different distance from the pivot hinge. As the cable from one gear shifter is moved, a linkage element is rotated and that movement is transferred to the cable for one derailleur. However, since the cable for the derailleur and the cable for the gear shifter lever connect to the linkage element at different points, only a percentage of the gear shifter lever""s movement is transferred to the derailleur through the cables. The result is that different gear shifter levers and derailleurs can be mechanically synchronized to work together on the same bicycle.